In telephone signalling equipment it is common to use a control signal at a single frequency, this signal being described as an SF signal, the frequency of this signal being "in-band" in the sense of being within the spectrum of voice frequencies transmitted over the telephone lines and through the other equipment and media comprising the entire system. A discussion of a typical system and this signal appears in U.S. Pat. No. 3,349,191, Mann, to which reference is made.
It is also common practice to employ echo suppression devices in telephone equipment for the general purpose of suppressing voice echoes which appear on the line and which tend to degrade the quality of the system. Such echoes are distracting and annoying to those using the equipment and become particularly noticeable and annoying when the path of travel of the signals through the transmission medium becomes quite long as, for example, in satellite systems and long-distance microwave systems.
Echo suppressors of this general type operate by comparing the voice signal levels on the two sides of the transmission system, i.e., the "transmit" side and the "receive" side with reference to a station or drop, and by opening or inserting a very high attenuation into that side which has the lower signal level. Thus, the assumption is made that the side carrying the voice will be seen by the comparing equipment to have the higher signal level and that any signal appearing on the other side is an echo and should be suppressed. This is basically a valid assumption and such equipment normally operates effectively.
However, when echo suppression is used, opening or greatly attenuating one line has the undesirable effect of removing the SF signal also, since it is in-band. In order to avoid this problem, it is customary to provide some local exchange equipment with apparatus for removing the SF signal from that portion of the line which includes an echo suppressor and the drop. Thus, the SF signal is removed from the line before it reaches the echo suppressor if one is used, and the voice signal attenuation accomplished by the echo suppressor therefore has no effect on the SF signal.
When station equipment is provided without echo suppression, it is necessary to insert echo suppression to permit use of the equipment in long distance service, but is is often not possible because of distance involved to insert such echo suppression between the SF equipment and the station. Alternatively, if neither is provided, it is necessary to provide both. In this case it would be necessary to provide bypass circuits around the ES unit, which can be done but only at considerable cost and waste of space.